A digital-to-analog converter (DAC) is a common element in many signal processing devices and systems, including for example many modern communication systems.
FIG. 1 illustrates an example of a simple and basic digital-to-analog converter DAC 100. DAC 100 includes a set of input buffers 102, a resistor ladder 104, an operational amplifier 106, and a feedback resistor RFB. In operation, DAC 100 receives an N-bit (e.g., N=7) digital input signal 105 and converts digital input signal 105 to an analog output signal 115. Although not shown in FIG. 1, in practice DAC 100 has associated therewith a reconstruction filter (e.g., a low pass filter) which may be included in DAC 100, or a separate external element, and may also have associated clocked input latches for digital input signal 105 to ensure that the bits from the LSB to the MSB transition at the same time. Many other DAC architectures are known, such as current-mode DACs which are typically employed for many high-speed, high-performance, DACs. FIG. 2 illustrates an example piecewise constant output signal of an idealized DAC before (trace 205) and after (trace 215) passing through a reconstruction filter.
To ensure that a DAC is able to meet its required performance characteristics, it is necessary to be able to accurately measure one or more performance characteristics, such as nonlinear distortion. Many modern digital-to-analog converters (DACs), such as DACs used in instrumentation and communications applications, operate at a very high speed with a very high degree of accuracy and high spectral purity. This in turn makes the requirements for measuring its performance more stringent and difficult to achieve.
Furthermore, to attain such high performance, it is sometimes desirable, or even necessary, to perform a calibration operation on the DAC, and to supply some adjustment or correction (e.g., predistortion) to the input signal provided to the DAC, or to one or more operating parameters of the DAC, so that it can meet its performance requirements. Performing calibration usually requires measuring the DAC output with higher accuracy than the target accuracy of the DAC itself. One possible solution is to perform measurements using laboratory equipment, such as an oscilloscope and/or a spectrum analyzer. While this is acceptable for debugging and characterizing a few experimental samples of a DAC (e.g., an integrated circuit including the DAC), requiring such test equipment to measure and calibrate each individual DAC in production would in general be prohibitively expensive. Employing off-chip test equipment for measurement (and possibly calibration) of DAC performance has other limitations in terms of the ability to examine internal DAC signals, undesirable filtering of the signals when routing them off-chip, the need for differential probes, and equipment limitations in terms of accuracy and/or the characteristics which they can easily measure.
As more DAC designs rely on calibration to achieve specified level of performance, some on-chip measurement techniques have been developed to characterize the behavior of DACs, providing information to the DAC calibration engine. However, directly oversampling the wideband output waveforms of high-speed DACs is inefficient, if even possible. Thus on-chip measurement circuits typically sample a subset of DAC output symbols. As long as the subset is representative, and corresponding digital DAC input sequence is known, the calibration can still be performed.
Meanwhile, previously proposed on-chip DAC calibrators only take samples of the DAC output waveform at integer multiples of the DAC sampling period (Ts). Thus, any spectral content above the first Nyquist zone gets aliased and corrupts the measurements. To minimize such error, (a) sampling should take place after a reconstruction filter, and (b) the reconstruction filter should be sharp enough to reject most of the spectral content above the first Nyquist zone. Both conditions are hard to meet, because (a) a reconstruction filter may need to be located off-chip for best performance, and (b) an on-chip filter may not be sharp enough to avoid significant aliasing artifacts. This is especially true for demanding wideband applications, such as instrumentation and communications.
Accordingly, it would be desirable to be able to provide an accurate measurement of one or more performance characteristics of a DAC, and possibly calibrating the performance of the DAC, without resorting to solutions which require off-device test equipment such as an oscilloscope, spectrum analyzer, etc.